The present invention relates to a method and to a device for obtaining a corrected servo control signal in a servo control loop.
A servo control loop is a closed loop used to keep an object moving along a track centered on that track by giving servo control signals to a positioning device which positions the object laterally to the track. A servo control signal may for different reasons be erroneous and appropriate measures must be taken to detect possible errors and to obtain the corrected servo control signal.
U.S. Pat No. 5,434,725, Hirose et al., discloses a device for positioning a magnetic head relative to a disc-shaped data carrier. The magnetic head reads servo data which is subsequently processed to obtain a servo control signal and a number. A comparator compares the number to a predetermined value and sets an error flag should the number be less than the predetermined value. The latter case is an indication for defective servo data. Normally, the servo control signal is used as an input for positioning the magnetic head but if the error flag is set the servo control signal is erroneous. The use of the erroneous servo control signal would possibly lead to a positioning of the magnetic head far off track, an error which is difficult to recover from. Instead a previous servo control signal, obtained from previously read and processed servo data is used to position the magnetic head. This compensates for the defective servo data and minimizes positioning errors.
The disclosed device handles errors due to defective servo data thereby taking into account an evolution of the servo control signal prior to the occurrence of defective servo data. It takes into account a dynamic behavior of the servo control signal by using the previous servo control signal instead of the erroneous one.
However errors due to defective servo data belong to only one type of error out of many possible types. Other occurring errors may be due to sudden peaks in the servo control signal. These peaks may be caused by an excessive deviation of a device reading the servo data away from e.g. a servo track containing the servo data. Excessive deviations typically happen when a sudden external mechanical force affects the device reading the servo data or perhaps a carrier carrying the servo data.
An aim of the present invention is to correct errors in the servo control signal which may be caused not only by defective servo data. The errors should be corrected taking into account the servo control signal""s dynamic.
In a first aspect of the invention a method for obtaining a corrected servo control signal in a servo control loop comprises reading servo data, processing read servo data to obtain at least a servo control signal, substituting a determined corrected servo control signal to the servo control signal if a value of processed read servo data overruns a determined threshold, the determined corrected servo control signal having a value proportional to the determined threshold. The method further comprises calculating a new threshold and substituting the new threshold to the determined threshold.
In a second aspect of the invention a flag is set if a value of processed read servo data overruns the determined threshold; The determined corrected servo signal is substituted to the servo control signal if the flag is set The new threshold is calculated as a function of the flag and a value of the servo control signal.
In a third aspect of the invention the calculating comprises calculating the new threshold from the determined threshold by increasing or decreasing the determined threshold depending on whether it is a maximum or a minimum respectively, if the flag is set. In the opposite case of the latter the calculating is done by increasing or decreasing the determined threshold if it is a maximum and if an absolute value of a difference between the determined threshold and the value of the servo control signal is smaller or greater than a determined xcex94-value respectively, and by increasing or decreasing the determined threshold if it is a minimum and if an absolute value of a difference between the determined threshold and the value of the servo control signal is greater or smaller than the determined xcex94-value respectively.
In a fourth aspect of the invention a minimum and a maximum respectively are used for the determined threshold.
In a fifth aspect of the invention a device for obtaining a corrected servo control signal in a servo loop comprises reading means to read servo data from a data carrier, processing means to obtain at least a servo control signal from read servo data, comparing means to compare a value of processed read servo data to a determined threshold, output means which generate a determined corrected servo control signal if a value of processed read servo data overruns the determined threshold, the determined corrected servo control signal being proportional to the determined threshold, updating means to calculate a new threshold and to substitute the new threshold to the determined threshold.
In a sixth aspect of the invention a servo loop positioning device for positioning an object relative to a predetermined track comprises a tracking head, to which the object is rigidly maintained, with reading means for reading servo data recorded along at least one servo track on a data carrier which is fixed in space relatively to the predetermined track, the servo track being parallel to the predetermined track, lateral positioning means for positioning the tracking head in a direction lateral to the servo track, signal processing means for processing read servo data, thereby determining a lateral position of the tracking head relative to the servo track and producing a servo control signal for the lateral positioning means such that the tracking head is positioned to be laterally centered on the servo track, comparing means to compare a value of processed read servo data to a determined threshold, output means which generate a determined corrected servo control signal if a value of processed read servo data overruns the determined threshold, and which feed the corrected determined servo control signal to the lateral positioning means by substituting it to the servo control signal, the determined corrected servo control signal being proportional to the determined threshold, and updating means to calculate a new threshold and to substitute the new threshold to the determined threshold.